week3 organel eng

8/23/2019 Week3 Organel ENG

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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

A Tour of the Cell

The three main parts of the Cell theory are:

(Schleiden and Schwann)

• All organisms are made up of one or more cells.

• The cell is the fundamental unit of structure andfunction in living things.

• All cells are essentially the same in chemicalcomposition.




• Robert Hooke, 1635-1703,İngilizMatematikçi,Fizikçi

Micrographia (1665) kitabında;geliştirdiği bileşik mikroskopla, inckesilmiş şişe mantarında boşluklarıgözlemlemiş ve onları “Hücre” olaraisimlendirmiştir.

Mikroskop, 2 kelimeden

oluşmaktadır.

"micro",küçük "scope" isenesnelere bakmaya

yarayan aygıt anlamına

gelmektedir.




1674 Leeuwenhoek protozoa’ ları incelemiştir.Dokuz yıl sonra ilk kez bakteri görmüştür.

• Antonie Leeuwenhoek (1632-1723), şarap gurmesi,vergimüfettişi, kumaş tüccarı

• 1677 Eritrositler ve spermkeşfi

• 1683 Bakterileri tanımlamış

• Kan dolaşımı teorisine katkıdabulunmuş.

• Tek mercekli mikroskobunkeşfi x200




• Light microscopes (LMs)

– Pass visible light through aspecimen

– Magnify cellular structures

with lenses

1 m

0.1 nm

10 m

0.1 m

1 cm

1 mm

100 µm

10 µ m

1 µ m

100 nm

10 nm

1 nm

Length of some

nerve and

muscle cells

Chicken egg

Frog egg

Most plant

and Animal cells

Smallest bacteria

Viruses

Ribosomes

Proteins

Lipids

Small molecules

Atoms

Nucleus

Most bacteria

Mitochondrion

L i g h t m i c r o s c o p e

E l e c t r o n m i c r o s c o p e

E l e c t r o n m i c r o s c o p e

Human height




– Use different methods for enhancing

visualization of cellular structuresTECHNIQUE RESULT

Brightfield (unstained specimen).

Passes light directly through specimen.

Unless cell is naturally pigmented or

artificially stained, image has little

contrast. [Parts (a) –(d) show a

human cheek epithelial cell.]

(a)

Brightfield (stained specimen).

Staining with various dyes enhances

contrast, but most staining procedures

require that cells be fixed (preserved).

(b)

Phase-contrast. Enhances contrast

in unstained cells by amplifying

variations in density within specimen;

especially useful for examining living,

unpigmented cells.

(c)

50 µm




Differential-interference-contrast (Nomarski).

Like phase-contrast microscopy, it uses optical

modifications to exaggerate differences in

density, making the image appear almost 3D.

Fluorescence. Shows the locations of specific

molecules in the cell by tagging the molecules

with fluorescent dyes or antibodies. These

fluorescent substances absorb ultraviolet

radiation and emit visible light, as shown

here in a cell from an artery.

Confocal. Uses lasers and special optics for

“optical sectioning” of fluorescently-stained

specimens. Only a single plane of focus is

illuminated; out-of-focus fluorescence above

and below the plane is subtracted by a computer.

A sharp image results, as seen in stained nervoustissue (top), where nerve cells are green, support

cells are red, and regions of overlap are yellow. A

standard fluorescence micrograph (bottom) of this

relatively thick tissue is blurry.

50 µm

50 µm

(d)

(e)

(f)


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Electron microscopes (EMs)

SEM provides for detailed study of the surface of a specimen

TECHNIQUE RESULTS

Scanning electron micro-

scopy (SEM). Micrographs taken

with a scanning electron micro-

scope show a 3D image of the

surface of a specimen. This SEM

shows the surface of a cell from a

rabbit trachea (windpipe) covered

with motile organelles called cilia.Beating of the cilia helps move

inhaled debris upward toward

the throat.

(a)

Cilia1 µm

Focus a beam of electrons through a specimen

(TEM) or onto its surface (SEM)



• The transmission electron microscope (TEM)

– Provides for detailed study of the internalultrastructure of cells

Transmission electron micro-

scopy (TEM). A transmission electron

microscope profiles a thin section of a

specimen. Here we see a section through

a tracheal cell, revealing its ultrastructure.

In preparing the TEM, some cilia were cutalong their lengths, creating longitudinal

sections, while other cilia were cut straight

across, creating cross sections.

(b)

Longitudinal

section of cilium

Cross section

of cilium 1 µm



Isolating Organelles by Cell Fractionation • Cell fractionation

– Takes cells apart andseparates the major

organelles from one

another

The centrifuge

Is used to fractionate

cells into their component parts

Tissue

cells

Homogenization

Homogenate1000 g

(1000 times the

force of gravity)

10 min Differential centrifugation

Supernatant poured

into next tube

20,000 g 20 min

Pellet rich in

nuclei and

cellular debris

Pellet rich in

mitochondria(and chloro-

plasts if cells

are from a

plant)

Pellet rich in

“microsomes”

(pieces of

plasma mem-

branes and

cells’ internal

membranes)

Pellet rich in

ribosomes

150,000 g 3 hr

80,000 g 60 min



Comparing Prokaryotic and Eukaryotic Cells • All cells have several basic features in common

– They are bounded by a plasma membrane

They contain a semifluid substance called

the cytosol

They contain chromosomes

They all have ribosomes



• Prokaryotic cells

– Do not contain a nucleus

– Have their DNA located in a region called

the nucleoid



(b) A thin section through the

bacterium Bacillus coagulans (TEM)

Pili: attachment structures on

the surface of some prokaryotes

Nucleoid: region where the

cell’s DNA is located (not

enclosed by a membrane)

Ribosomes: organelles that

synthesize proteins

Plasma membrane: membrane

enclosing the cytoplasm

Cell wall: rigid structure outsidethe plasma membrane

Capsule: jelly-like outer coating

of many prokaryotes

Flagella: locomotion

organelles of some bacteria

(a) A typical

rod-shaped bacterium

0.5 µm Bacterial

chromosome



• Eukaryotic cells

– Contain a true nucleus, bounded by amembranous nuclear envelope

– Are generally quite a bit bigger than

prokaryotic cells




• A animal cell

Rough ER Smooth ER

Centrosome

CYTOSKELETON

Microfilaments

Microtubules

Microvilli

Peroxisome

Lysosome

Golgi apparatus

Ribosomes

In animal cells but not plant cells:

Lysosomes

CentriolesFlagella (in some plant sperm)

Nucleolus

Chromatin

NUCLEUS

Flagelium

Intermediate filaments

ENDOPLASMIC RETICULUM (ER)

Mitochondrion

Nuclear envelope

Plasma membrane




• A plant cell

In plant cells but not animal cells:

Chloroplasts

Central vacuole and tonoplast

Cell wall

Plasmodesmata

CYTOSKELETON

Ribosomes (small brwon dots)

Central vacuole

Microfilaments

Intermediate

filaments

Microtubules

Rough

endoplasmic

reticulum Smooth

endoplasmic

reticulum

Chromatin

NUCLEUS

Nuclear envelope

Nucleolus

Chloroplast

PlasmodesmataWall of adjacent cell

Cell wall

Golgi apparatus

Peroxisome

Tonoplast

Centrosome

Plasma membrane

Mitochondrion




Relative Volumes Occupied by the MajorIntracellular Compartments

INTRACELLULAR COMPARTMENT PERCENTAGE OF TOTAL CELL

VOLUME

Cytosol 54

Mitochondria 22

Rough ER cisternae 9

Smooth ER cisternae plus Golgi cisternae 6

Nucleus 6

Peroxisomes 1

Lysosomes 1

Endosomes 1




C

e l l N u c l e

u s

DNA



Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 18

Structure of the Nucleus

Nucleus:

• largest organelle

Stores genetic information as DNA

Contains genetic information for making proteins

Controls all cellular activities and protein synthesis

• Nuclear envelope:

– double membrane around the nucleus, connected to ER

• Nuclear pores with regulator proteins:

– Control exchange of materials between cytoplasm

and nucleus




nuclear lamina

an attachment sitefor…

b) The continuity of the outer nuclear membrane with ER. So the space between the inner and outer nuclear membranes is directly connected with the lumen fo the ER.

c) The inner nuclear membrane is lined by the nuclear lamina, which serves as an attachment site for chromatin (nuclear lamina)

N l L i




Nuclear Lamina►Meshwork of intermediate filaments

►Consists of "intermediate filaments", 30-100 nm thick.

►These intermediate filaments arepolymers of lamin, ranging from 60-75 kD.

►A-type lamins are inside, next tonucleoplasm; B-type lamins are near the

nuclear membrane (inner). They may bindto integral proteins inside that membrane.

►Maintenance of nuclear shape

►Spatial organization of nuclear pores

►Regulation of transcription►Anchoring of interphase chromatin

►DNA replication




Within the Nucleus

• Nucleoplasm:

– fluid containing ions, proteins

(enzymes), DNA, RNA, andnucleoli

• Nucleolus: Dark areas

– site of rRNA synthesis andpackaging into ribosomalsubunits

– Contains rRNA (ribosomalribonucleic acid), proteins

(85%) and ribosomal DNA




The Nucleolus and rRNA Processing

• Nucleolus: Site for rRNA transcription, processing,

some aspects of ribosome assembly.

• Actively growing mammalian cells have 5 to 10 x 106 ribosomes, must be synthesized each time cell divides.

• Nucleolus is not surrounded by a membrane

• All cells contain multiple copies of rRNA genes (ex.oocytes)

• If removed, the cell can not divide

• Chro 13,14,15, 21 & 22 in charge

• Nucleolar organizing regions (NORs): The areas for ribosome RNA are located and

They can synthesize the 28S, 18S,

and 5.8S rRNA for ribosome, 5S from nucleus




Structure of nucleolus

• Heterochromatin is highly condensed transcriptionally inactive




Heterochromatin(dark staining)

and euchromatin

(bright staining)

Heterochromatin

Euchromatin

• Heterochromatin is highly condensed, transcriptionally inactive• Euchromatin is decondensed, distributed throughout

• Basic dyes

(hematoxylin)

http://tureng.com/search/hematoxylin

http://tureng.com/search/hematoxylin




DNA replication:

• Mammalian cells: clustered sites from labeling newlysynthesized DNA with bromodeoxyuridine (BrdU in place of T)

newly replicated DNA in discrete cluster s

A: early replication

B, late replication

O i ti f DNA




Organization of DNA

• Chromatin is composed of DNA, histone, nonhistone protein,and some RNA

• DNA in chromatin is organized into Nucleosomes (DNA coiledaround histones)

• During Nuclear Division, Chromatin is tightly coiled into visiblechromosomes (23 pairs in humans)

Chromosome

• Chromosomes are rod-like, coiled structures

• Chromosomes remain condensed and visible in the nucleus justprior to cell division.

• Humans have 46 chromosomes except in gamete cells.

Function: Contains all the genetic information in

triplet codes. e.g. CAT GAG TCA.




Histone:Histone is positively charged and contains arginine and lycine.

Histone protein is synthesized during the S phase only.

Histones can be sorted as two types:

1. Highly conserved core histone including H2A, H2B, H3, and H4.2. Non conserved linker histone including H1 only.

The core histone is highly conserved, especially the H4 is.

NonHistone Protein: Negatively charged and acidic nonhistone protein

binds to the specific DNA sequence of chromosome.Nonhistone protein can be synthesized during the whole cell cycle

The functions of nonhistone are as the follows:

• Help DNA molecules to form different structure domains that are

beneficial to DNA replication and gene transcription.

• Help to start DNA replication reactio (topoisomerase II)• Regulate transcription and gene expression.

Basic histone protein makes an easy binding to negative DNA




From DNA to chromosome:

If you open and extend the DNA molecule in each chromosome;

It will be 5cm long.

If you link all DNA molecules in a nucleus together,

it will be 1.7 – 2.0 m long.

But, the diameter of nucleus is shorter than 10 μ m.

Nucleosome (110A0 in dia.):

Nucleosome is a beaded structure composed of core particles and linker DNA. Nucleosome can be described :

•Each nucleosome includes about 200bp DNA, one histone core, and

an H1.

•The octameric histone core is composed of 8 molecules from H2A,H2B, H3, and H4 by two molecules from each.

•DNA molecule winds the core particle with a left hand helix and

80bp for each circle. 1.75 circles for each structure

• Adjacent core particles are linked by a 60bp linker DNA

Structures of nucleosome




Structures of nucleosome

Ribosomes




Ribosomes – Are particles made of ribosomal RNA

and protein

– the site of protein synthesis in the cell

– found within the cytosol of the cytoplasm andattached to internal membranes

Ribosomes Cytosol

Free ribosomes

Bound ribosomes

Large

subunit

Small

subunit

TEM showing ER and ribosomes Diagram of a ribosome

0.5 µm

Ribosomes




Ribosomes

• Made of 2 sub-units, rRNA (ribosomal RNA) & proteins,therefore they are not membrane-bound.

• rRNA sub-unit is produced by the nucleolus.• They may be free, i.e. suspended in the cytoplasm or

attached to the rough endoplasmic reticulum.

• 65% RNA, 35% protein

• Mito &Chloroplast have their own ribosomes

• Function: Site of protein synthesis

1) RER ribosomes are for ER, Golgi, Secretion & integral membrane

proteins,2) Free ribosomes are for cytoplasmal & peripheral proteins

Ribosome assemblyFormation of ribosomes requires assembly of pre-rRNA

with ribosomal proteins, then export of subunit




1 = large sub-unit (+49 protein)

2 = small sub-unit (+33 protein)

mRNA

Synthesized

protein

Polysomes




Polysomes

• A linear collection of several ribosomesattached to one mRNA.

• Function: produces many copies ofpolypeptides when attached to the mRNA,

e.g. the protein pigment Hb (hemoglobin)

E d b S t




Endomembrane System

Surrounds

cytoplasm

How it works




How it works….

• *The following membrane-bound structures,nucleus /nuclear envelope , ER (rough and smooth),

Golgi bodies , vesicles , vacuoles and lysosomes ,have a functional interrelationship. They functiontogether in the synthesis and transport ofmolecules within the cell or for export out of thecell.

Endomembrane System (Golgi Ap ER &Lysosomes)




Smooth ER

Rough ER

ER lumen

Cisternae

Ribosomes

Transport vesicleSmooth ER

Transitional ER

Rough ER 200 µm

Nuclear

envelope

Endomembrane System (Golgi Ap, ER &Lysosomes) • The endoplasmic reticulum (ER) (5-6nm in dia.)

– Accounts for more than half the totalmembrane in many eukaryotic cells

Is continuous with the

nuclear envelope

There aretwo distinct regions of ER:SER (no riosome)& RER (bound ribosome)

Functionsof Smooth ER




Functions of Smooth ER

– Synthesizes fosfolipids, ceramide &steroids (enzymes in the membrane)

– Glycogen metabolism

– Stores calcium (muscle)

– Detoxification (poisons/drugs/cholesterolusing cytochrome p450)

– Establishing resistance to medicine (newborn baby)

– NOTE : We see, in common, in liver, testis,ovary, kidney, stomach, striated musclecells, eye (excess give shape)

Functionsof Rough ER




Functions of Rough ER

• Ribosomes attached (also to nuclear outermembrane)

• Produce proteins (soluble, integral &secreted)

• which are then transported throughout thecell in tubules/canals to be exported out

• Attachment of carbohydrate especially for

secreted proteins (Glycolysation)• Membrane assembly

• Folding (ex.:lysosome proteins), quality control &

degradation

The Golgi Apparatus: Shipping & Receiving Center




– Receives many of the transport vesicles

produced in the rough ER

– Consists of flattened membranous sacs called

cisternae

The Golgi Apparatus: Shipping & Receiving Center

Functions of the Golgi apparatus include:

• Modification of the products of the rough ER

-( N-linked glycosylation (N-acetyl glucoseamine, fucose,

galactose, N-acetyl neuraminic acid),

- adding phosphates to mannose residues,- adding sulphate)

• Maturation of proteins

• O-linked glycosylation

• Manufacture of certain macromolecules




Golgi

apparatus

TEM of Golgi apparatus

cis face

(“receiving” side of

Golgi apparatus)

Vesicles movefrom ER to GolgiVesicles also

transport certain

proteins back to ER

Vesicles coalesce toform new cis Golgi cisternae

Cisternal

maturation:

Golgi cisternae

move in a cis-

to-trans

direction

Vesicles form and

leave Golgi, carrying

specific proteins to

other locations or to

the plasma mem-

brane for secretionVesicles transport specific

proteins backward to newer

Golgi cisternae

Cisternae

trans face

(“shipping” side of

Golgi apparatus)

0.1 0 µm 1 6

5

2

3

4

• Functions of the Golgi apparatus




Return to the ER

Lysome

To plasma membrane for

secretion

Interior of Golgiapparatus

1. In the endomembrane

system, proteins bound for

different destinations are given

different carbohydrate "tags."

2. Proteins are sorted in the

Golgi apparatus.

3. Transport vesicles bud from

the Golgi apparatus and travel to

their destinations.

4. Proteins on vesicle surfaceinteract with receptors at

destination. 5. Vesicle delivers contents.

Protein Secretion

Vesicles




Vesicles

• Cellular processes which make use of vesicles includes(example):

– autolysis, – Secretion (enzyme, hormone & proteins, etc.)

– intracellular digestion,

– Pinocytosis & phagocytosis, – packaging of neurotransmitters & viruses,

– storage of Ca 2+ in muscle cells or macromolecules inliver & muscle cells & plant cells,

– detoxification of alcohol & drugs into H2O-solubleproducts,

– breaking down H2O2 (hydrogen peroxide: a toxicbyproduct made by many enzymes) into H2O + O2 via the

enzyme catalase (i.e. in peroxisomes) located in the liver




Lysosomes (0 2 06 )



Lysosomes (0,2-06mm)

• The interior of these structures, is more acidic than the rest of the cytoplasm, is filled with large numbers of small granules,which are protein aggregates of as many as 40 differenthydrolase (digestive) enzymes

• Most abundantly found in lung, spleen and WBC• The lysosomes provide an intracellular digestive system that

allows the cell to digest; damaged cellular structures, foodparticle & unwanted matter such as bacteria.

• Lysosomal storage diseases (I- cell Disease, Tay Sach(hexominidase)

• Large, irregular structuressurrounded by single membrane,

form by breaking off from the Golgi apparatus

•Glycoprotein rich interior

membrane

FORMATION OF LYSOSOMES



FORMATION OF LYSOSOMES

AND LYSOSOMAL ENZYMES

• Lysosomal enzymes produced on membrane- boundribosome.

• They are channel into matrix of endoplasmic

reticulum.• Goes to golgi appratus

• Package in the form of secretory granules or “primarylysosomes”

• Then fuse to vacuole (food, microorganism byendocytosis, pinocytosis & phagocytosis) to becomesecondary lysosome

Note: not found in plant & not found in erytrocyte




• Lysosomes carry out intracellular digestion by

Phagocytosis

(a) Phagocytosis: lysosome digesting food

1 µm

Lysosome contains

active hydrolytic

enzymes

Food vacuole

fuses withlysosome

Hydrolytic

enzymes digestfood particles

Digestion

Food vacuole

Plasma membrane

Lysosome

Digestive

enzymes

Lysosome

Nucleus




L y s o s o m e : A u t o

p h a g y

These are all

part of

endomembrane

system

Vacoule




Vacoule

• Central vacuoles

– Are found in plant cells – Hold reserves of important

organic compounds and water

Central vacuole

Cytosol

Tonoplast

Central

vacuoleNucleus

Cell wall

Chloroplast

5 µm

• Vacuoles are formed by phagocytosis

• A larger membrane-enclosed sac.

• Pigment Vacuole (pigment) and ContractileVacuole (expel water)

Function

• Storage of macromolecules such as a foodvacuole formed by phagocytosis.

Relationships among organelles of the




Plasma membrane expands

by fusion of vesicles; proteins

are secreted from cell

Transport vesicle carries

proteins to plasma

membrane for secretion

Lysosome available

for fusion with another

vesicle for digestion

4 5 6

Nuclear envelope is

connected to rough ER,which is also continuous

with smooth ER

Nucleus

Rough ER

Smooth ERcis Golgi

trans Golgi

Membranes and proteins

produced by the ER flow in

the form of transport vesicles

to the GolgiNuclear envelop

Golgi pinches off transport

Vesicles and other vesicles

that give rise to lysosomes and

Vacuoles

1

3

2

Plasma

membrane

Relationships among organelles of the

endomembrane system




• Mitochondria and Chloroplasts change energy

from one form to another

• Mitochondria

– Are the sites of cellular respiration

– Are found in nearly all eukaryotic cells

• Chloroplasts

– Found only in plants, are the sites of

photosynthesis

Mitochondria (except erytrocytes) 0 5 m




Mitochondria (except erytrocytes), 0,5mm

Traditionally not considered to be

part of endomembrane system

Mitochondria -1




Mitochondria 1• The “ powerhouse ” of the cell

– Membrane-bound organelle with its own DNA

(deoxyribonucleic acid, i.e. mDNA (mitochondrial DNA) – Contains 2 membranes:

– an inner folded (70% protein) /convoluted membrane calledcristae &

– a smooth outer membrane (Porin, 5kDa, enzymes for lipid synthesis) .

– Cristae; increases surface area for cellular respiration andthe production of ATP with the aid of several proteins and

complexes (e.g. ATP complexes, transport proteins/electrontransport chains).

– The matrix is the inner fluid-filled space containing DNA,RNA, ribosomes, proteins, enzymes, H+/protons etc. (Note:membranes are also composed of phospholipids and proteins)

Mitochondria -2




The inner membane proteins:• Cytochorome

• Permease (transport in & out metabolites)• ATP synthase (FoF1ATP synthase)

• Phosphate Translocase (H2PO4- & H+)

• Electron exchanger (such as malate aspartate exchanger

for NADH)

M tochondr a

Matrix:Enzymes for Crebs Cycle(TCA), pruvate & fat acid oxidation

Excess Ca+2 storage

Note: urea synthesis in liver cells, partial steroid synthesis &

heme synthesis for Hemoglobin

Mitochondria 2




Mitochondria -2

MtDNA• 2-10 circular double helix DNA (light & heavy)• About 17Kb

• Self replication (any time) & transcription

• 13 protein, 22 tRNA & 2 rRNA (cytochrome oxidase,

NADH dehydrogenase• No packing with histone as chromosomal DNA

• Almost completely coding

• Codon show difference in comparison to nuclear codon.

Ex: UGA for trytophan, instead of stop codon

• Maternal inheritance (not much from paternal)

• No quality control for DNA replicaion error (10 times higher

mutation rate in comparison to nuclear)

• Mutation can be homoplasmic or heteroplasmic

Peroxisome (Microbodies, 0,1-1mm)




( )

• All animal cells (except erythrocytes) contain peroxisomes

• Roughly spherical & single membrane-enclosed organelles

• Contains 50 different enzymes

• Do not contain DNA or ribosomes

• Peroxisome protein synthesized on free ribozymes

• Not part of the endomembrane system

• Contain “Oxidative enzymes” such as catalase, glucose-urate & d-amino acid oxidase

• Peroxisome is the source of H2O2

* glyoxysomes in plants contain enzymes for

converting fats to carbohydrates

FUNCTIONS OF PEROXISOME




FUNCTIONS OF PEROXISOME

• Detoxification (breakdown the excess fatty acids (the very long

one), uric acid, amino acid, methanol)

• Accelerate gluconeogensis from fats (by enzymes)

• Degrade purine to uric acid (bile acid),

• The site of oxygen utilization in the cell

• Cholesterol, bile acid & ether lipid (plasmalogen) biosynthesis

Centrosomes and Centrioles




– Centrosome is considered to be a “microtubule-organizing

center

– Contains a pair of centriolesCentrosome

Microtubule

Centrioles

0.25 µm

Longitudinal section

of one centrioleMicrotubules Cross section

of the other centriole

Centrosomes




Also called the centrosphere or cell center, which refersto a specialized zone of cytoplasm containing the centrioles

and a variable number of small dense bodies calledcentriolar satellites.

Considered to be a center of activities associated with cell

division, usually adjacent to the nucleus. The Golgi apparatus often partially surrounds the

centrosome on the side away from the nucleus.

Plant and fungal cells have a structure equivalent to a centrosome, although they do not contain centrioles .

entrosomes




Centrioles are self-duplicating organelles that exhibit continuity from one cell generation to the next.

They double in number immediately before cell division Paired centrioles are called diplosome.

Microtubule organizing centers (MTOCs) become nucleationsites around each centriole to form the fibers of the aster andthe mitotic spindle.

MTOCs determine cell polarity including the organization of cell organelles, direction of membrane trafficking, and

orientation of microtubules. Because microtubule assembly is nucleated from MTOCs,

the (-) end of most microtubules is adjacent to the MTOC& the (+) end is distal.

The matrix of the centrosome isorganized by a pair of centrioles



organized by a pair of centrioles.

The ring of modified microtubulesof the centriole is visible,

surrounded by the fibrouscentrosome matrix.

In EM, each centriole is found to bea hollow cylinder, closed at one end

& open at the other

In transverse section, its wall iscomposed of 9 evenly spacedtriplet microtubules (9x3



A centrosome with

attached

microtubules. Theminus end of each

microtubule isembedded in the

centrosome, havinggrown from a Ý-tubulin ring

complex, whereasthe plus end of

each microtubule is free in thecytoplasm.

Th t i l d th t f th t



The centrioles and other components of the centrosome areduplicated in interphase cells,

But they remain together on one side of the nucleus until the

beginning of mitosis. The two centrosomes then separate and move to oppositesides of the nucleus, forming the two poles of the mitoticspindle.

As mitosis proceeds, the two chromatids of each chromosome

are then pulled to opposite poles of the spindle. This chromosome m ovement is mediated by m otor proteins

associated with the spindle microtubules.

After cell division, each cell acquires 2 centrioles, one from theparent cell, and one which arose as a procentriole.

If mitotic cells are exposed to drugs like colchicine (binds tomonomeric tubulin and prevent polymerization), vinblastine

and taxol (disrupt microtubule dynamics), microtubulesdisappear and mitosis is arrested because of inadequate

formation of the mitotic spindle.These drugs are useful in the treatment of certain cancers.



• Cells rely on the integration of structures

and organelles in order to function

5 µ m

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